A power converter is a power processing circuit that converts an input voltage or current source waveform into a specified output voltage or current waveform. A full-bridge phase-shift pulse-width-modulated power converter (hereinafter referred to as a FPP converter) is a frequently employed switched-mode power converter that converts a direct current (DC) input waveform to a specified DC output waveform. The FPP converter generally includes switching circuitry coupled to an input source of electrical power. The switching circuitry includes two pairs of alternately conducting active switches. A primary winding of a transformer is coupled to the switching circuitry and a secondary winding of the transformer is coupled to a rectifier circuit (e.g., rectifying diodes). The rectifier circuit is coupled through an output filter to a load.
While the FPP converter employs the leakage inductance of the transformer to achieve zero-voltage switching (ZVS) across the active switches, other sources of inefficiencies exist in the FPP converter. More specifically, a parasitic capacitance in the form of the winding capacitance in the transformer and junction capacitance of the rectifying diodes resonate with the leakage inductance thereby inducing transients (e.g., ringing and voltage spikes) in the secondary side of the FPP converter. The transients are intensified in higher power and current applications. The transients are especially harmful to the rectifier circuit and noticeably affect the overall efficiency of the FPP converter.
There have been attempts in the past to minimize the effects of transients in power converters and the resulting stress on rectifier circuits. For instance, a primary clamping circuit that includes a pair of clamping diodes and an auxiliary inductor on the primary side of the transformer is disclosed in A Novel Soft-Switching Full-Bridge DC/DC Converter: analysis, design considerations and experimental results at 1.5 kW, 100 kHz, by R. Redl, N. O. Sokai and L. Balogh, IEEE Power Electronics Specialists Conf. Rec., p. 162-172 (1990) which is herein incorporated by reference. In general, the leakage inductance of the transformer is minimized and, with the assistance of the auxiliary inductor, ZVS is achieved across the switching circuitry. The voltage across the primary winding of the transformer is clamped by the clamping diodes at the line voltage of the power converter or the ground. As a result, the resonance between the auxiliary inductor and the parasitic capacitance in the windings of the transformer does not induce excessive voltage stress across the rectifier circuit. While in theory the reduction in voltage stress appears viable, the primary clamping circuit does not actually minimize the transients across the rectifier circuit. While the voltage across the primary winding is clamped at or near the input voltage, the voltage in the secondary side of the power converter still rings due to the resonance between the leakage inductance and parasitic capacitance of the transformer and the junction capacitances of the diodes. Therefore, the primary clamping circuit is ineffective in reducing the voltage stress across the rectifier circuit, especially in higher power applications.
A resistor-capacitor-diode (RCD) snubber circuit is disclosed in A 1 kW, 500 kHz Front-End Converter for a Distributed Power Supply System, by L. H. Mweene, C. A. Wright and M. F. Schlecht, Proc. IEEE Applied Power Electronics Conf., p. 423-432 (1989) which is herein incorporated by reference. A RCD snubber circuit is coupled to the rectifier circuit in the secondary side of the transformer. As opposed to disposing the energy diverted from the rectifier circuit within the power converter itself, the RCD snubber circuit attempts to divert the energy to the output of the power converter. While the RCD snubber circuit is a relatively simple design, the circuit suffers from substantial power dissipation across the resistor that substantially affects the overall efficiency of the power converter. As the current through the power converter increases, the losses escalate thereby limiting the RCD snubber to low power applications.
Another component readily employed to reduce the voltage stress on a rectifier circuit is a saturable reactor. A saturable reactor circuit is disclosed in An Improved Zero-Voltage-Switched Pulse-Width-Modulated Converter Using a Saturable Inductor, by G. Hua, F. C. Lee and M. M. Jovanovic, IEEE Power Electronics Specialists Conf. Rec., p. 189-194 (1991) which is herein incorporated by reference. Conventionally, a saturable reactor is series-coupled to each rectifying diode of the rectifier circuit. While the saturable reactor does a relatively good job of limiting the voltage stress across the rectifier circuit, the saturable reactors exhibit losses that result in a relatively high temperature rise across the core and windings thereof. The temperature rise can be alleviated by employing several saturable reactors in parallel, but at the cost of valuable space on the printed circuit board and a prohibitively expensive saturable reactor circuit. As previously mentioned, the aforementioned circuits and other prior art circuits have inadequately dealt with the transients that adversely effect the rectifier circuit in power converters.
Accordingly, what is needed in the art is a snubber circuit for a rectifier circuit that minimizes the voltage stress thereacross to reduce the power losses associated with the rectifier circuit and oscillations in both voltage and current therefrom and is suitable for a vast range of power applications including higher power applications.